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Nystagmus is involuntary eye movement can be part of the vestibulo-ocular reflex (VOR), with the eyes moving first in the direction of the lesioned side (slow phase) followed by a quick correction (fast phase) to the opposite side or away from the lesioned side.The direction of nystagmus is defined by the direction of its quick phase (e.g. a right-beating nystagmus is characterized by a rightward-moving quick phase). The oscillations may occur in the vertical, horizontal or torsional planes, or in any combination. The resulting nystagmus is often named as a gross description of the movement, e.g. downbeat nystagmus, upbeat nystagmus, seesaw nystagmus, periodic alternating nystagmus. These descriptive names can be misleading however, as many were assigned historically, solely on the basis of subjective clinical examination, which is not sufficient to determine the eyes' true trajectory. Over the past forty years, however, objective eye movement recording techniques have been applied to the study of nystagmus, and the results have led to a greater accuracy and understanding of the condition.

Nystagmus is not to be confused with other superficially similar-appearing disorders of eye movements (saccadic oscillations) such as opsoclonus or ocular flutter that are composed purely of fast-phase eye movements, while nystagmus is characterised by the combination of a slow eye movement that usually act to take the eye off the point of regard, interspersed with the fast movement that serves to bring the eye back on target. Without the use of objective recording techniques, it may be very difficult to distinguish between these conditions.

In medicine, the presence of nystagmus can be benign, or it can indicate an underlying visual or neurological problem. Over forty types of nystagmus have been classified.

In police work, testing for horizontal gaze nystagmus is one of a battery of field sobriety tests used by officers in the field to determine whether a suspect is driving under the influence of alcohol. The test involves observation of the suspect's pupil as it follows a moving object, noting (1) lack of smooth pursuit, (2) distinct and sustained nystagmus at maximum deviation, and (3) the onset of nystagmus prior to 45 degrees. A general rule of thumb is that a person's blood alcohol concentration can be estimated by subtracting the angle of onset from 50 degrees. Therefore, a person with an angle of onset of nystagmus at 35 degrees has a blood alcohol concentration of approximately 0.15%. While there is some scientific basis behind this obervation, in practice the test is fatally flawed, for it relies on subjective examination and interpretation of eye movements, and estimation of gaze angles, which are notoriously inaccurate.

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Induced nystagmus

An easy way of inducing nystagmus is by having the person close her or his eyes and spin around. After a few spins, there is a distinct jerking of the eyes from side to side when they are reopened: this is rotatory-induced nystagmus. The degree of nystagmus varies greatly between people and even in the same person at different times.

Another type of induced nystagmus is the optokinetic nystagmus (OKN). It can be elicited by presenting a moving pattern. The eyes tend to track the pattern, but snap back regularly. The nystagmus persists for a short while even after the cessation of the stimulus. This is called "opto-kinetic after-nystagmus" (OKAN).

If the pathologic nystagmus is based in the central nervous system (CNS), such as with a cerebellar problem, the nystagmus can be in any direction including horizontal. Purely vertical nystagmus is usually central in origin.

Vestibular nystagmus usually combines a rotational component with vertical or horizontal eye movements and may be spontaneous or positional.

Horizontal nystagmus is also classified into three degrees as follows:

First degree nystagmus is present only on lateral gaze, and has the fast phase in the direction of gaze;

Second degree nystagmus is present in the primary (neutral) position of gaze;

Third degree nystagmus is present in the same direction in all gaze positions

Such distinctions help to identify the anatomical source of the nystagmus. First degree nystagmus usually originates in the brainstem or cerebellum, while second and third degree nystagmus are usually vestibular in origin.

Causes

Congenital nystagmus occurs more frequently than acquired nystagmus, is not associated with other disorders (such as refraction errors or diplopia) and is usually mild and non-progressive. The affected persons are not aware of their spontaneous, small-amplitude eye movements.

Diagnosis

Nystagmus can be clinically investigated by using a number of non-invasive standard tests. The simplest one is to irrigate an external auditory meatus with warm or cold water. The temperature gradient provokes the stimulation of the vestibulocochlear nerve and the consequent nystagmus.
The resulting movement of the eyes may be recorded and quantified by special devices called electronystagmograph (ENG), which is a form of electrooculography (an electrical method of measuring eye movements using external electrodes) or even less invasive devices called videoonystagmograph (VNG), which is a form of videooculography(VOG) (a video-based method of measuring eye movements using external small cameras built into head masks). Special swinging chairs with electrical controls are also used in this test to induce rotatory nystagmus.

Therapy

Congenital

Congenital nystagmus has traditionally been viewed as non-treatable, but medications have been discovered in recent years that show promise in some patients. In 1980, researchers discovered that a drug called baclofen could effectively stop periodic alternating nystagmus. Subsequently, gabapentin, an anticonvulsant, was found to cause improvement in about half the patients who received it to relieve symptoms of nystagmus. Other drugs found to be effective against nystagmus in some patients include memantine, levetiracetam, 3,4-diaminopyridine, 4-aminopyridine, and acetazolamide.[1] Several therapeutic approaches, such as contact lenses, drugs, surgery, and low visionrehabilitation can also be used in order to improve visual function.

Acquired

Pathological acquired nystagmus is mostly a temporary condition and stops spontaneously. When it is secondary to a neurological disorder, this must be treated accordingly.